The immune response occurs only if a T lymphocyte has been able to recognize the foreign antigen in association with a self molecule encoded in the major histocompatibility complex (MHC). These self molecules expressed on antigen- presenting cells are the ploymorphic class II MHC antigens. In man, three class II MHC antigens exist: HLA-DR, -DQ and -DP; each one consists of an alpha-beta chain dimer. The aim of this project is to define whether additional class II antigens exist and to analyze how class II antigens function in antigen presentation and in their interaction with T lymphocytes. Two new functional class II genes have been found: DZ alpha and DO beta. Their distinct pattern of expression suggests that they do not encode alpha-beta chain partners and that each one may correspond to a new class II antigen. By DNA-mediated gene transfer into murine fibroblasts it was shown that an alpha chain from one subregion (e.g. DR) could assemble and be expressed at the cell surface together with beta chains from other subregions (DO, DP and DQ) and vice- versa. These mixed isotype pairs could contribute significantly to the nonallelic diversity of MHC class II antigens and to the repertoire of restricting elements that can be used in immune responses. Cytotoxic T cells (CTL) specific for measles virus- infected cells are predominantly class II-restricted. Transfection of the measles virus matrix and nucleocapsid genes into fibroblasts previously transfected with DR alpha and beta genes has led to two new findings: (1) class II-restricted CTL do not require intact cell surface viral antigens for recognition and lysis and, (2) newly synthesized cytoplasmic proteins can be presented by class II antigens. Thus, antigen-presentation pathways for class I and class II antigens may not be mutually exclusive (collaboration with S. Jacobson). Transfection experiments which introduced the human CD4 molecule into a murine hybridoma specific for a murine class I MHC antigen and DR antigen into the murine target cell provided strong evidence that a direct CD4-DR interaction occurs and results in enhanced T cell response.